This invention relates generally to microwave frequency multiplexers, particularly those of a comb-line type.
Multiplexers are utilized for dividing a common wide band signal into a plurality of distinct adjacent frequency bands. For example, consider a particular microwave circuit carrying a broad band signal that extends from 1500 MHz to 3000 MHz but which carries four separate signals independent of each other by a technique termed frequency multiplexing. By that technique, one of the four separate signals would be carried in the frequency band of 1500 to 1850 MHz, the second in the adjacent but distinct frequency band of 1850 to 2200 MHz, the third 2200 to 2600 MHz and the fourth 2600 to 3000 MHz. This single broad band signal is separated into its individual signals, by one known technique, by using a plurality of parallel connected bandpass microwave filters all having their inputs connected to that single circuit but being tuned to pass only one of the distinct signal carrying bands having a 350 or 400 MHz bandwidth. The four individual signals are separately recovered at the outputs of the bandpass filters.
Such a multiplexer is preferably made of a plurality of parallel connected comb-line filters. The comb-line type of microwave bandpass filter is generally preferred because of its ease of manufacture and wider range of tolerances than other types of microwave filters. Each of the inputs of the individual comb-line bandpass filters is connected to the single broad band signal input terminal in a manner, and with other components, so that the operation of one filter does not adversely affect the operation of the other.
The input admittance (reciprocal of impedance) of a multiplexer needs to be matched to the broad band signal circuit and remain uniform over the entire bandwidth of the circuit and filter. Maintaining the real part of the input admittance of existing multiplexers at a uniform level over this frequency range is not a particular problem, but maintaining the imaginary component of the input admittance at a uniform level is extremely difficult with existing techniques, especially when the bandwidth of the multiplexer is very large and it includes a large number of individual parallel filters. Since most circuits to which the multiplexer input is connected have an impedance with a zero imaginary admittance component over its bandwidth, such a requirement is generally the case for the input admittance of a multiplex filter as well. Otherwise, energy is undesirably reflected from the filter input. This reflection is indicated by the voltage-standing-wave-ratio (VSWR) quantity. These requirements, and several existing approaches to satisfying those requirements, are suggested by G. L. Matthaei and E. G. Cristal in an article entitled "Theory and Design of Diplexers and Multiplexers," appearing at pages 237-326 of a book Advances in Microwaves, Volume 2, edited by Leo Young, and published by Academic Press in 1967. However, the approaches to providing a multiplexer having an imaginary part of the input admittance being substantially equal to zero over the full bandwidth of the multiplexer are not adequate for many present requirements for a multiplexer having a plurality of separate bandpass filters and designed to operate in the microwave frequency range over 100 MHz and having a band edge frequency ratio exceeding 2 to 1.
Therefore, it is a principal object of the present invention to provide an improved microwave multiplexer structure and method of multiplexing that maintains the input VSWR within acceptable limits over those broader bandwidths at such microwave frequencies.
It is another object of the present invention to provide such a device and technique that are simply realizable and easily implemented in compact units.